Process for unsaturated aldehyde oxidation using a supported catalyst

ABSTRACT

An improved process for the conversion of an unsaturated aldehyde (such as acrolein) to the corresponding unsaturated carboxylic acid (such as acrylic acid) is disclosed. The improved process comprises reacting in the gas phase at a temperature sufficient to accomplish the desired reaction of the aldehyde with oxygen in the presence of unagglomerated particles of a supported catalyst having the empirical formula Mo a  V b  W c  Mn d  O e , the atomic ratio of Mo:V:W:Mn:O being such that when a is 12, b is 0.5 to 12, c is 0.1 to 6, d is 0.5 to 20 and e is 37 to 94, the catalytic metals being supported on porous silica particles having a surface area of from about 25 to about 350 m 2  /gm and a porosity of from about 0.2 to about 1.0 cc/gm whereby essentially all of the catalytic metals are contained on the surfaces of the particles.

BACKGROUND OF THE INVENTION

Numerous processes and catalysts are known in the prior art for theproduction of unsaturated, carboxylic acids by the oxidation ofunsaturated aldehydes, e.g., the production of acrylic acid fromacrolein. A very efficacious process of this type is shown in U.S. Pat.No. 3,644,509 which discloses producing an unsaturated carboxylic acid,such as acrylic acid, by reacting the corresponding unsaturatedaldehyde, such as acrolein, with oxygen in the presence of a catalysthaving the empirical formula Mo_(a) V_(b) W_(c) Mn_(d) O_(e), the atomicratio of MO:V:W:Mn:O being such that when a is 12, b is 0.5 to 12, c is0.1 to 6, d is 0.5 to 20 and e is 37 to 94.

The search has continued for improved catalysts and processes to produceunsaturated carboxylic acids by the oxidation of unsaturated aldehydes.

Metal oxide catalysts are often used "neat" (i.e., unsupported) inprocesses of this type and often as particulates in a bed or zone withina reaction vessel. Generally, the reactants are introduced, eitherseparately or concurrently, at one end of the zone and flow co-currentlythrough the reaction zone in contact with the particular catalyticmaterial therein with the reaction product stream (including the desiredproduct) exiting from the zone at the end opposite introduction.

A number of difficulties have been found to arise in the operation ofsuch neat metal oxide-catalyzed processes. It has been found, forexample, that substantially (i.e, more than 75 percent) of the totalconversion occurs in the first portion of the catalyst zone or bed(i.e., the first 25 percent of the total catalyst zone or bed) which isin contact with the reactants. The concentration of conversion in thefirst portion of the zone in an exothermic reaction raises the exothermtemperature at that point substantially in excess of that at laterpoints in the catalyst zone. These high exotherm temperatures makecontrol of the reaction more difficult, dictate more expensiveheat-resistant materials, often decrease the yield of desired productand/or increase the yield of undesired by-products and are otherwisedisadvantageous.

Generally, attempts to solve the problems associated with such aconcentration of conversion in the initial portion of a neat catalystzone have focused on diluting the catalyst in such a manner so as toessentially homogenize the exotherm temperature throughout the length ofthe catalyst zone. Dilution has been attempted both by adding separatediscrete particles of a non-catalytic material uniformly dispersed withthe catalyst particles and by mixing the catalyst material with thenon-catalytic material (in solution, slurry or the like) and forming theresulting mixture into relatively homogeneous particles of an admixtureof catalytic and noncatalytic material for use in the catalytic reactionzone.

These types of dilution of catalytic material in a reaction zone havebeen found, however, to manifest other problems. That is, it isfrequently considerably more expensive to prepare the diluted catalystzones or beds. In addition, product yields from such diluted catalystzones often are lower both in total product yield and productselectivity than with the corresponding neat catalyst zones.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a process for theconversion of an unsaturated aldehyde to the corresponding unsaturatedcarboxylic acid which reduces or alleviates the problems of the priorart.

It is a specific object of the present invention to provide a processfor the conversion of an unsaturated aldehyde to the correspondingunsaturated carboxylic acid which maintains a relatively even exothermtemperature throughout the reaction zone, the maximum exothermtemperature being 30°C.

It is also an object of this invention to provide a process for theconversion of an unsaturated aldehyde to the corresponding unsaturatedcarboxylic acid which provides a high yield of and selectivity for thedesired product.

It is further an object of this invention to provide a process for theconversion of an unsaturated aldehyde to the corresponding unsaturatedcarboxylic acid which conversion is relatively inexpensive to perform.

It is further an object of this invention to provide a process for theconversion of an unsaturated aldehyde to the corresponding unsaturatedcarboxylic acid utilizing a single form of catalytic componentthroughout substantially all of the catalytic reaction zone.

The present invention provides an improved process for the conversion ofan unsaturated aldehyde to the corresponding unsaturated carboxylic acidwhich comprises reacting in the gas phase at a temperature sufficient toaccomplish the desired reaction of an aldehyde of the following formula:##EQU1## wherein R₁ is hydrogen or an alkyl radical of from 1 to 6carbon atoms and R₂ and R₃ are hydrogen or methyl radicals; with oxygenin the presence of a supported catalyst having the empirical formulaMo_(a) V_(b) W_(c) Mn_(d) O_(e), the atomic ratio of Mo:V:W:Mn:O beingsuch that when a is 12, b is 0.5 to 12, c is 0.1 to 6, d is 0.5 to 20and e is 37 to 94, the improvement which comprises using as the catalystunagglomerated particles of the above empirical formula supported onporous silica particles having a surface area of from about 25 to about350 m² /gm and a porosity of from about 0.2 to about 1.0 cc/gm, thecatalytic metals being contained essentially on the surfaces of theparticles.

The essence of the present invention is the discovery that a particulartype and composition of support material for this particular catalystmaterial used in this particular reaction offers a surprising balance ofhigh yield and high selectivity of desired product while maintaining areasonably even exotherm temperature along the reaction zone. Inaddition, it has been found that the improved process of the presentinvention can be performed with lower amounts of catalyst per mole ofreactants than the corresponding neat catalyst process withoutdecreasing product yield or selectivity.

DESCRIPTION OF PREFERRED EMBODIMENTS

The catalyst as described above and as used in the present invention maybe regarded as a mixture of oxides of the various metals and/or mixturesof heteropoly acid salts of the various metals (both of which mixturesare hereinafter called "catalytic metals") supported on the externalsurfaces of an unagglomerated porous silica support material.

That is, once the various catalytic metals (in the form of oxides orsalts) are applied onto the porous silica support particles (in a manneras described hereinbelow), the particles are not agglomerated, compactedor otherwise physically combined into larger-sized particles. In thismanner, essentially all of the catalytic metals remain disposed on theexternal surfaces of the porous silica support material. Such externalsurfaces include the outer (e.g., peripheral) surfaces and also some orall of the surfaces of the pores in the particles. In the particles usedin the process of the present invention, however, essentially all of thecatalytic metals are exposed to the vapor phase reactants andessentially none of the catalytic metals are disposed within thenon-porous interior of a particle (as occurs when catalyst-containingparticles are thereafter agglomerated into larger particles).

Since the catalyst particles as used in the process of the presentinvention are not agglomerated or otherwise further combined intolargersized particles, the size and shape of the silica supportparticles on which the catalytic metals are deposited is essentiallydetermined by the size and shape of the ultimate catalyst particlesdesired. The particle shape and size of the porous silica particles willvary somewhat depending upon the particular reaction, reaction vesseland the like, and it will be understood that the porous silica particles(and the resulting catalyst particles) may be of any size and shapeeffective to catalyze a particular reaction. Particle size can be, forexample, from about 0.01 to about 1 inch. Generally, for commericalreactors the porous silica particles (and the resultingcatalyst-containing particles) will be spherical, cylindrical, orelliptical in shape. Spherical particles of this type generally willhave a diameter in the range of from about 1/8 to about 1/2, preferablyfrom about 1/4 to about 3/8 inch. Cylindrical- (or elliptical-) shapedparticles of this type will generally have their diameter (or bothdiameters for elliptical-shaped particles) in the range of from about1/8 to about 1/2, preferably from about 1/4 to about 3/8, inch and alength of from about 1/8 to about 1/2, preferably from about 1/4 toabout 3/8, inch.

The porous silica particles useful as the catalyst support have asurface area of from about 25 to about 350, preferably from about 100 toabout 200, m² /gm and a porosity of from about 0.2 to about 100,preferably from about 0.3 to about 0.8, cc/gm. Porosity is measured inaccordance with conventional techniques known in the art such as mercuryporosimetry.

The catalyst may be prepared by any suitable technique for depositingthe catalytic material onto the surfaces of the support but ispreferably prepared by depositing the metal oxide compounds or theirprecursors on and in the porous silica particles and calcining theparticles at a temperature of about 200° to 600°C. or more in thepresence of oxygen. For example, aqueous solutions of water-solublecompounds of the metals may be mixed, the porous silica particles addedthereto and the water removed by evaporation or the like to leave dryparticles suitable for calcining. Suitable water-soluble compoundsuseful in the preparation of a catalyst in accordance with the foregoingmethod include ammonium paramolybdate, ammonium metavanadate, ammoniumparatungstate, manganous acetate, ammonium metatungstate, orthotungsticacid, metataungstic acid, molybdic acid, molybdenum pentoxide,molybdenum trioxide, manganous benzoate, and manganese nitrate.

The catalyst particles are preferably formed by first forming an aqueoussolution of water-soluble salts of molybdenum, vanadium and tungsten,adding the porous silica particles thereto to absorb the solution,adding a solution of a manganese salt which causes precipitation of themetal compounds, drying and calcining the particles. Suitablewater-soluble manganese compounds useful in the preparation of acatalyst in this manner include the water-soluble manganese salts of aninorganic or organic acid. Preferably the manganese salts of C₁ to C₈carboxylic acids or nitric acid are utilized. The water-solublecompounds of molybdenum, vanadium and tungsten useful in this method ofpreparation of the catalyst particles again include ammoniumparamolybdate, ammonium metavanadate, ammonium paratungstate, manganousacetate, ammonium metatungstate, orthotungstic acid, metatungstic acid,molybdic acid, molybdenum pentoxide and molybdenum trioxide. Theammonium salts of these metals are preferred for use in this method ofcatalyst preparation.

As noted above, the supported catalyst has the empirical formula Mo_(a)V_(b) W_(c) Mn_(d) O_(e), the atomic ratio of Mo:V:W:Mn:O being suchthat when a is 12, b is 0.5 to 12, c is 0.1 to 6, d is 0.5 to 20 and eis 37 to 94. In a particularly preferred catalyst, the atomic ratio ofMo:V:W:Mn:O is such that when a is 12, b is 1 to 6, c is 0.3 to 3.0, dis 1 to 12 and e is 40 to 84. Generally, the catalyst particles containfrom about 5 to about 75, preferably from about 25 to about 60, mostpreferably from about 35 to about 50, weight percent of the catalystmaterials and concommitantly from about 95 to about 25, preferably fromabout 75 to about 40, most preferably from about 65 to about 50, weightpercent of the porous silica support material.

The present process may be carried out continuously or non-continuouslyand the catalyst may be present in various forms such as in one or morefixed beds or as a fluidized system.

Portions of the reactants which do not undergo reaction may be recycledif desired. The temperatures employed are preferably between 250° and325°C. although higher or lower temperatures generally between 200° and350°C. may be employed.

The pressure utilized in the present process may be subatmospheric,atmospheric or superatmospheric. Usually pressures ranging from 0.5 to3.0 atmospheres will be utilized although pressure up to 10 atmospheresand higher may be suitably employed. The contact time of the reactantswith the catalyst at the reaction conditions is generallly between 0.3and 15 seconds but is preferably a relatively short time of from 0.5 to10 seconds. By contact time as used herein is meant the contact timeadjusted to 25°C. and atmospheric pressure (conditions denoted by NTP).Thus, the contact time is calculated by dividing the volume of thecatalyst bed (including voids) by the volume per unit time flow rate ofthe reactants at NTP.

The oxygen necessary as a reactant in the present process may be frompractically any molecular oxygen-containing gas such as concentratedmolecular oxygen or air. Also, the molecular oxygen-containing gas maybe one wherein molecular oxygen is mixed in varying amounts with aninert diluent gas such as nitrogen, argon, or a carbon oxide. Theunsaturated aldehyde reactant may be premixed with the oxygen-containinggas before introduction into the reaction zone or the reactants may beintroduced separately into the reaction zone. Also, the unsaturatedaldehyde and/or molecular oxygen may be introduced into the reactionzone at one or a plurality of points along the length of the reactionzone. The reactants may be pretreated before entering the reaction zonesuch as for the removal of undesirable components therefrom.

Although other unsaturated acyclic aldehydes may be oxidized to thecorresponding carboxylic acids by the present process, the aldehydespreferably have αβ -unsaturation with the most suitable aldehydes beingin of the formula: ##EQU2## wherein R₁ is hydrogen or an alkyl radicalof 1 to 6 carbon atoms and wherein R₂ and R₃ are hydrogen or methylradicals. Preferably the present process is utilized for the productionof acrylic acid from acrolein. Other conversions that may beaccomplished include methacrolein to methacrylic acid, andcrotonaldehyde to crotonic acid.

In conducting the oxidation reaction, the gaseous feed mixture shouldgenerally contain from 0.5 to 6 moles of oxygen per mole of theunsaturated aldehyde although the preferable range is from 1.0 to 4.0moles per mole. Water is also desirably present in the gaseous feed inamounts of from 1 to 25, preferably 2 to 20, moles per mole ofunsaturated aldehyde. In addition to water, diluents which are gaseousunder the reaction conditions and are relatively inert may be introducedinto the system. Suitable diluents include nitrogen, flue gas, CO₂ andparaffinic hydrocarbons.

While unsaturated aldehydes as described above are commericallyavailable, it is advantageous to utilize as the starting materialherein, the reaction product stream from a reaction zone in which analkene of the following formula: ##EQU3## wherein R₁ is hydrogen or analkyl radical of from 1 to 6 carbon atoms; and R₂ and R₃ are hydrogen ormethyl radicals; is catalytically oxidized to the correspondingunsaturated aldehyde. Such an alkene oxidation process may be conductedin any suitable manner as known in the art.

When the alkene oxidation product stream is utilized as the startingmaterial in the present invention it is preferred that the alkeneoxidation to unsaturated aldehyde reaction zone and unsaturated aldehydeoxidation to unsaturated carboxylic acid reaction zone be maintainedserially such that the reaction product stream from the alkene oxidationzone may be utilized directly (i.e., without intermediate separationand/or product recovery) as the starting unsaturated aldehyde feedmaterial in the unsaturated aldehyde oxidation zone.

The unsaturated carboxylic acid product resulting from the aldehydeoxidation process of the present invention is useful in a number ofindustrial uses and particularly as a starting material for the ultimateproduction of plastic resin materials such as acrylic resins,methacrylate resins, methyl methacrylate resins and the like.

The invention is additionally illustrated in connection with thefollowing Examples which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Examples.

EXAMPLE 1

Solutions of 15.79 grams of ammonium molybdate in 56 cc water, 2.65grams of ammonium metavanadate in 56 cc water and 2.23 grams of ammoniumparatungstate in 56 cc water are mixed together at 70°C. 32.5 grams ofsilica particles are added while stirring the solution. The silica has asurface area of 150 m² /gm, a particle size of -20+30 mesh (about 0.03to 0.02 inch) and a porosity of 0.51 cc/gm measured by mercuryporosimetry. A solution of 5.50 grams of manganese acetate is added tothe mixture and the resulting slurry is evaporated to dryness in astream of dry air. The solid material thus obtained is calcined in airat 400°C. for 5 hours. The composition of the resulting catalyst isrepresented by the formula Mo₁₂ V₃ W₁.2 Mn₃ O₅₃ /SiO₂. The catalystcontains 65 percent by weight silica and 35 percent by weight ofcatalytic material.

A 5 cc sample of the above-produced -20+30 mesh particle size catalystis tested for the oxidation of acrolein to acrylic acid in a laboratoryreactor which consists of a stainless steel U-tube of 0.364 inchinternal diameter heated in a fluidized sand bath. A gaseous mixture of2.6 mole percent acrolein, 45.2 mole percent steam and 52.2 mole percentair is passed over the catalyst with an NTP contact time of 0.87seconds. The liquid product collected and the vent gases are eachanalyzed by gas chromatography. The results obtained are given below,where:

Conversion = 100 × (Total moles of carbon recovered --moles of carbonrecovered as acrolein)/(Total moles of carbon recovered)

Selectivity = 100 × (Moles of carbon in product as acrylic acid/(Totalmoles of carbon recovered -- Moles of carbon recovered as acrolein)

Yield = 100 × (Moles of carbon in product as acrylic acid/Total moles ofcarbon recovered)

          Reaction                                                                Run No.                                                                             Temperature, °C.                                                                 Conversion %                                                                         Selectivity %                                                                         Yield %                                        __________________________________________________________________________    1     285        99    95      95                                             2     285        98    96      94                                             3     290       100    95      95                                             4     290       100    96      96                                             __________________________________________________________________________

EXAMPLE 2

A catalyst of the same chemical composition as Example 1 is prepared bythe procedure described in Example 1 except that the silica has aparticle size of -20 to +30 mesh, a surface area of 60 m² /gm and aporosity of 0.69 cc/gm. The catalyst is tested for acrolein oxidationunder the conditions described in Example 1, with the following results:

          Reaction                                                                Run No.                                                                             Temperature, °C.                                                                 Conversion %                                                                         Selectivity %                                                                         Yield %                                        __________________________________________________________________________    5     314       100    91      91                                             6     315       100    92      92                                             7     315        99    92      92                                             8     314       100    92      92                                             __________________________________________________________________________

This Example serves to illustrate the importance of the surface area andpore volume of the support on the activity and selectively of thecatalyst. The catalyst of Example 1 employs a relatively high surfacearea silica, and shows higher activity (as evidenced by the temperaturenecessary to operate at 100 percent acrolein conversion since the lowerthe operating temperature the more active the catalyst), selectivity andyield than the catalyst of Example 2 which employs a silica of lowersurface area.

COMPARATIVE EXAMPLE A

A catalyst of the same chemical composition as that in Example 1 isprepared using the metal salts described therein. The silica is added inthe form of 108.3 grams of a 30 weight percent aqueous colloidalsuspension of silica. After evaporating to dryness and calcining in themanner described in Example 1, the fine particles are agglomerated tospherical particles of -20+30 mesh size. The agglomerated catalystparticles prepared in this manner each appear (upon visual andmicroscopic examination) to be a uniform dispersion of catalytic andnon-catalytic (i.e., silica) material. A substantial portion of thecatalytic metals appear to be disposed within the solid portion of thebody of the particles so as not to be exposed to vapor phase reactants.

The resulting catalyst particles have a particle size of about -20 to+30 mesh, a surface area of 93m² /gm and a porosity of 0.34 cc/gm. Thecatalyst is tested for acrolein oxidation under the conditions describedin Example 1 with the following results:

          Reaction                                                                Run No.                                                                             Temperature, °C.                                                                 Conversion %                                                                         Selectivity %                                                                         Yield %                                        __________________________________________________________________________     9    299       84     95      80                                             10    315       96     93      90                                             11    315       96     94      90                                             12    322       98     94      91                                             13    322       98     93      91                                             14    330       98     92      90                                             __________________________________________________________________________

This Example serves to illustrate the importance of the use of acatalyst particle containing the catalytic metals on the externalsurfaces of the finished catalyst. The catalyst of this Example which,because of the colloidal size of the silica and the subsequentagglomeration after deposition of the catalytic metals, containscatalytic metals in its interior portions not exposed to the reactants,shows lower activity, as evidenced by the higher operating temperaturerequired for complete acrolein conversion, also lower selectivity andyield than the catalyst of Example 1 employing the preferred method ofpreparation.

COMPARATIVE EXAMPLE B

Comparative Example A is repeated except that the silica is added in theform of fine particles all smaller than 30 mesh having a distributiongenerally of from about -30+ 250 mesh (with about 15 weight percent ofthe particles going through the 250 mesh screen) and with an averageparticle size of about 100 microns, a surface area of 70 m² /gm and aporosity of 0.69 cc/gm. The silica used is the silica of Example 2ground to the finer size. After evaporating to dryness and calcining inthe manner described in Example 1, the particles are consolidated intoparticles of -20+ 30 mesh size. The catalyst is tested for acroleinoxidation under the conditions described in Example 1, with thefollowing results:

          Reaction                                                                Run No.                                                                             Temperature, °C.                                                                 Conversion %                                                                         Selectivity %                                                                         Yield %                                        __________________________________________________________________________    15    315       94     93      88                                             16    315       93     93      87                                             17    325       98     91      89                                             18    325       91     92      83                                             19    325       96     92      88                                             20    335       100    88      88                                             21    335       100    91      91                                             __________________________________________________________________________

This Example serves to illustrate again the importance of utilizingunagglomerated catalyst particles containing the catalytic metals on thesurfaces of the particular silica support particles. The acroleinconversion process using the catalyst prepared by the method of Example1 shows higher activity, selectivity and yield than the process usingthe catalyst of this Example prepared as described in this other manner.

COMPARATIVE EXAMPLE C

A catalyst is prepared by the method of Example 1 except that the silicasupport is not included. The chemical composition of the Mo-V-W-Mnoxides in the unsupported catalyst of this Example and the supportedcatalyst of Example 1 are identical.

Acrolein oxidation is performed in accordance with the procedure ofExample 1 utilizing the above-prepared unsupported catalyst as well asthe catalyst of Example 1 and Comparative Example A. The temperatureprofile through the catalyst bed in each run is measured using a 5junction thermocouple arranged along the vertical axis of the catalystbed both in the center of the bed and along the exterior wall of thereaction zone. The difference between the bed temperature and walltemperature at a given point is the exotherm temperature. The maximumexotherm temperature for each run and other result are given below:

                                  Maximum                                                                       Exotherm                                               Temper-                                                                             Conversion                                                                          Selectivity                                                                          Yield                                                                             Temperature                                     Catalyst                                                                             ature, °C.                                                                   %     %      %   °C.                                      __________________________________________________________________________    Comparative                                                                   Example C                                                                            247   100   93     93  41                                              Example 1                                                                            285   100   97     97  15                                              Comparative                                                                   Example A                                                                            335   100   92     92  31                                              __________________________________________________________________________

This Example illustrates that the acrolein oxidation process of thepresent invention utilizing the particular unagglomerated silicasupported catalysts give the highest selectivity and yield to acrylicacid and significantly moderate the temperature in the catalyst bed.They are superior in these respects to unsupported catalysts(Comparative Example C) and an agglomerated, supported catalyst(Comparative Example A). The decrease in the exotherm generated in thecatalyst bed is very significant in operation of a catalyst, asundesirable temperature changes may result in catalyst deactivation orphysical deterioration and loss in efficiency to desired products. Inaddition, analysis of the product streams in each run shows that theacrolein oxidation process of the present invention producessubstantially less propionic acid as an impurity than the run utilizingthe catalyst of Comparative Example C. As known in the art, propionicacid is a detrimental impurity in acrylic acid particularly when theacrylic acid is further utilized to form plastic resin materials. Forsuch use, the acrylic acid should contain as small an amount ofpropionic acid as possible.

EXAMPLE 3

A solution containing 136.1 grams ammonium molybdate, 22.85 gramsammonium metavanadate and 19.23 grams ammonium paratungstate in 1450 ccwater is concentrated by evaporating off excess water until its volumeis 200 cc. 280 grams of silica in the form of 5 mm spheres are added tothis solution, which is totally absorbed by the silica pellets. Thesilica has a surface area of 150 m² /gm. and a porosity of 0.51 cc/gm.47.45 grams of manganese acetate dissolved in a minimum amount of wateris then added. The catalyst is dried at 120°C. and calcined in air at400°C. for 5 hours. The catalyst contains 33 weight percent catalystmaterial and 67 weight percent silica support.

A liter of catalyst is placed in a 1 inch O.D. steel reactor tube heatedat 286°C. A gaseous feed mixture containing 4.1 mole percent acrolein,48.0 mole percent nitrogen, 6.6 mole percent oxygen and 41.3 molepercent water is passed over the catalyst with a contact time of 2.48seconds. 86 percent of acrolein fed is converted with 86 percentefficiency to acrylic acid.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. In a process for the conversion of an unsaturatedaldehyde to the corresponding unsaturated carboxylic acid whichcomprises reacting in the gas phase at a temperature sufficient toaccomplish the desired reaction an aldehyde of the following formula:##EQU4## wherein R₁ is hydrogen or an alkyl radical of from 1 to 6carbon atoms and R₂ and R₃ are hydrogen or methyl radicals; with oxygenin the presence of a supported catalyst having the empirical formulaMo_(a) V_(b) W_(c) Mn_(d) O_(e), the atomic ratio of Mo:V:W:Mn:O beingsuch that when a is 12, b is 0.5 to 12, c is 0.1 to 6, d is 0.5 to 20and e is 37 to 94, the improvement which comprises using as the catalystunagglomerated particles of the above empirical formula supported onporous silica particles having a surface area of from about 25 to about350 m² /gm and a porosity of from about 0.2 to about 1.0 cc/gm, thecatalytic metals being contained essentially only on the surfaces of theparticles, and wherein said process is performed in a reaction vesseland the maximum exotherm temperature is 30°C.
 2. The improved processaccording to claim 1 wherein said temperature is within the range offrom 200° to 350°C.
 3. The improved process according to claim 1 whereinwater is present in an amount of from 1 to 25 moles per mole ofunsaturated aldehyde.
 4. The improved process of claim 1 wherein saidaldehyde is acrolein or methacrolein, wherein the temperature is withinthe range of about 200° to 350°C., the pressure is within the range ofabout 0.5 to 10 atmospheres, wherein water is present in amounts of fromabout 1 to 25 moles per mole of said aldehyde and wherein oxygen ispresent in amounts of from about 0.5 to 6.0 moles per mole of saidaldehyde.
 5. The improved process of claim 1 wherein said silicaparticles have a surface area of from about 100 to about 200 m² /gm anda porosity of from about 0.3 to about 0.8 cc/gm.
 6. The improved processof claim 1 wherein when a is 12, b is 1 to 6, c is 0.3 to 3.0, d is 1 to12 and e is 40 to
 84. 7. The process of claim 6 wherein said silicaparticles have a surface area of from about 100 to about 200 m² /gm anda porosity of from about 0.3 to about 0.8 cc/gm.
 8. The improved processof claim 1 wherein said aldehyde is formed in a first reaction zone bythe catalytic oxidation of an alkene of the following formula: ##EQU5##wherein R₁, R₂ and R₃ are defined as above.
 9. A process for theconversion of acrolein to acrylic acid comprising reacting in the gasphase acrolein with oxygen in the presence of a metal oxide catalyst ofthe empirical formula

    Mo.sub.a V.sub.b W.sub.c Mn.sub.d O.sub.e

the atomic ratio of Mo:V:W:Mn:O in said catalyst being such that when ais 12, b is from 1 to 6, c is from 0.3 to 3.0, d is from 1 to 12 and eis from 40 to 84, said metal oxide catalyst being contained andsupported essentially only on the surfaces of porous silica particles ofup to about 1 inch in maximum dimension, said porous silica particleshaving a surface area of from about 100 to about 200 m² /gm and aporosity of from about 0.3 to about 0.8 cc/gm, said process beingconducted at temperatures from about 250°C to 325°C, pressures from 0.5to 3.0 atmospheres, water being present in amounts of from 1 to 25 molesper mole of acrolein, and wherein said process is performed in areaction vessel and the maximum exotherm temperature is 30°C.
 10. Aprocess according to claim 9 wherein water is present in an amount offrom 2 to 20 moles per mole of acrolein.